Data transmission method and radio system

ABSTRACT

The invention relates to a data transmission method and radio system. In particular, the invention relates to a method and system in which both the GMSK and the m-PSK modulation methods are available for the signal to be transmitted an in which the modulation method used at a given time can be changed to another modulation method. In the solution of the invention, the m-PSK modulator of the transmitter includes a multiplier adapted to multiply the signal to be transmitted by a given coefficient making the signal constellations of the received signal similar regardless of the modulation method. Thus, the processing of the signal in the receiver becomes easier and the quality of the transmission improves in connection with m-PSK modulation.

This application is the national application PCT/F199/00606 filed Jul.8, 1999 which designated the U.S.

FIELD OF THE INVENTION

The invention relates to a data transmission method and radio system. Inparticular, the invention relates to a method and system in which boththe GMSK and the m-PSK modulation method are available for the signal tobe transmitted and in which the modulation method used at a given timecan be changed to another modulation method.

BACKGROUND OF THE INVENTION

In radio telecommunication systems the quality of the channel, i.e. theradio path, varies continuously. Several factors affect the quality of aradio system channel, for instance multipath propagation, fading,interference in the environment and many other matters.

When designing known radio systems, the aim has been to secure thequality of the signal also when the quality of the channel is poor. Whendesigning data transmission systems, an essential parameter is themodulation method used in the transmission path. The information symbolsto be transmitted cannot as such be transmitted over the transmissionpath due to the dissipation occurring in the transmission path and thecapacity of the transmission path, and the symbols must be modulated bya suitable method to produce a satisfactory transmission path capacityand transmission quality. When designing known systems, the main focusin selecting the modulation method has been to secure the transmissionquality making the performance of the modulation methods in poor channelconditions very important. This is why the ability of current systems totransmit signals having high data rate is rather poor. When securing thequality of the transmission, the capacity has suffered.

An example of known modulation methods is GMSK (Gaussian Minimum ShiftKeying), which is used in the GSM (Global System for MobileCommunication) cellular radio system. It has a limited frequencyspectrum and its performance is good, but the data transmission ratesare not very high. The continuous phase shift keying methods m-PSK makeit possible to attain high data rates, but the modulation method workswell only when the transmission channel does not have much interference,i.e. the signal-to-noise ratio is good.

One solution for optimizing performance and transmission rate is tochange the modulation method used according to current needs. The GMSKmethod can be used when good interference tolerance is required, andwhen the channel quality is good, for instance the 8-PSK method can beused to achieve three times as fast a data rate as compared with GMSK.

The problem with known radio systems is how to change the modulationmethod seamlessly during an ongoing connection. Change of modulationmethod causes problems especially in the receiver, because thetransmitter can change its modulation method without notifying thereceiver in advance. A seamless change of modulation method is, however,needed in packet switched data transmission, for instance.

Therefore, when using the GMSK and PSK methods, the receiver must whenreceiving a signal check and decide which of the modulation methods hasbeen used in sending the signal. As the modulation method can changewithout prior notice, this checking of modulation method is a continuousoperation, and the easier it is done, the better. Previously, thedecision on the modulation method used was made on the basis of thetraining sequence of each frame. As the signal constellation of areceived GMSK signal rotates in the receiver, the rotation must beremoved before detection. The signal constellation of a PSK signal doesnot rotate in the receiver, so the signals must be treated in differentways before channel estimation. This causes problems, because beforechannel estimation the receiver does not know the modulation methodused.

BRIEF DESCRIPTION OF THE INVENTION

Thus, the object of the invention is to implement a method and a radiosystem implementing the method in a manner that the above-mentionedproblems are solved. This way, the receiver can effectively demodulateand detect the transmitted signal even though it does not know themodulation method of the transmitter in advance. This is achieved by thedata transmission method of the invention, in which method both the GMSKand the m-PSK modulation method are available for the signal to betransmitted and the modulation method used at a given time can bechanged to another modulation method. In the method of the invention,when using the m-PSK modulation method, the signal is multiplied by agiven coefficient making the received signal constellations similarregardless of the modulation method.

A further object of the invention is a radio system comprising a set oftransmitters and receivers, of which transmitters at least some compriseboth a GMSK modulator and a m-PSK modulator to modulate the signal and acontrol unit adapted to select the modulation method used at a giventime. In the system of the invention, the m-PSK modulator of thetransmitter comprises a multiplier adapted to multiply the signal to betransmitted by a given coefficient which makes the signal constellationsof the received signal similar regardless of the modulation method.

The method and system of the invention provide several advantages.Different modulation methods can be seamlessly combined in the solutionof the invention, because the signals transmitted by differentmodulation methods can be treated in the same way before channelestimation.

In a preferred embodiment of the invention, the m-PSK signal to betransmitted is multiplied by a coefficient which makes the signalconstellation in the receiver rotate in the same way as theconstellation of a GMSK-modulated signal. With the solution of theinvention one disadvantage of the PSK modulation method is avoided,which occurs when the data to be transmitted contains only zero bits. Insuch a case, the modulated signal comprises only the carrier wave, butin the solution of the invention, this situation is avoided due to therotation of the constellation.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in greater detail inconnection with preferred embodiments and with reference to the attacheddrawings, in which

FIG. 1 shows an example of a radio system of the invention,

FIG. 2 illustrates the structure of a transceiver,

FIG. 3 illustrates a signal constellation of the GMSK modulation method,

FIG. 4 illustrates a signal constellation of the 8-PSK modulationmethod,

FIG. 5 illustrates the structure of a modulator,

FIG. 6 illustrates the structure of a PSK modulator, and

FIG. 7 illustrates the structure of a demodulator.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a digital data transmission system to which thesolution of the invention can be applied. The system in question is apart of a cellular radio system which comprises a base station 100 whichhas a bi-directional connection 102 to 106 to subscriber terminals 108to 112. Further, the base station has a connection to a base stationcontroller 114 which switches the connections of the terminals elsewherein the network. Both the GMSK and the m-PSK modulation methods areavailable for the terminals and the base station of the system, or atleast a part of this equipment. The modulation method used in a givensituation at a given time can be changed to another modulation method.With poor signal-to-noise ratio, the GMSK method can be used and whenthe transmission path allows it, the 8-PSK method, for instance. EDGE,i.e. a further developed system of the digital GSM cellular radiosystem, is a good example of a system of the invention. However, theinvention is not limited to it, as is obvious to those skilled in theart. The invention can also be used in other systems.

Let us next examine an example of the structure of a transmitter in asystem of the invention. Here, the structure of a subscriber terminal108 is presented. Corresponding components are also substantially foundin a transceiver of a base station, as is obvious to those skilled inthe art. In the receiving direction, a signal received by an antenna 200is forwarded to a duplex filter 202 which separates the frequencies usedin transmission and reception from each other. From the duplex filter202, the signal is forwarded to radio frequency parts 204 in which thesignal is converted to an intermediate frequency or directly to a baseband, and the thus converted signal is sampled and quantized in ananalogue/digital converter 206. From the converter, the signal isforwarded to an equalizer 208 which compensates interference, such asthat caused by multipath propagation. The modulation is demodulated in ademodulator 210, i.e. a bit stream is extracted from the equalizedsignal and forwarded to a demultiplexer 212. The demultiplexer 212separates the bit stream from different time-slots into their ownlogical channels. From the demultiplexer, the signal is forwarded tode-interleaving and decipherment 213. After this, a channel codec 214decodes the bit stream of different logical channels, i.e. decideswhether the bit stream is signalling data to be forwarded to a controlunit 216, or speech to be forwarded to a speech codec 218 Which decodesspeech coding. From the speech codec, the signal is forwarded on to aloudspeaker 220. The channel codec 214 also corrects errors. The controlunit 216 performs internal control tasks by controlling various units.

In the transmitting direction, the signal is forwarded from a microphone222 to the speech codec 218 which does speech coding. From the speechcodec, the signal is forwarded to the channel codec 214 which doeschannel coding. The data coming out from the channel codec 214 isinterleaved and enciphered 219. After this, the signal is forwarded to aburst builder 224 which forms the burst to be transmitted for instanceby adding a training sequence and tail bits to the data coming from thechannel codec 218. A multiplexer 226 indicates a time-slot for eachburst. A modulator 228 modulates the digital signals to a radiofrequency carrier. This is described in greater detail later on in thedocument. The modulated signal is forwarded to a radio frequencytransmitter unit 230 in which the signal is filtered beforetransmission, i.e. the bandwidth of the signal is limited to a requiredarea, and after the filtering the signal is transmitted through theduplex filter 202 by means of the antenna 200. The transmitter 230 alsocontrols the output power of the transmission. A synthetizer 232arranges the necessary frequencies for the various units.

The presented equipment can also comprise other components, such asadapters and filters, as is obvious to those skilled in the art.

Let us next examine more closely the operation of the modulator 228. Inthe following, 8-PSK is used as an example of m-PSK. However, theinvention is not limited to it, as is obvious to those skilled in theart.

FIG. 3 illustrates a signal constellation of a received signal when thesignal has been transmitted by the GMSK modulation method. In principle,the constellation comprises four points 300, 302, 304 and 306. Let usassume that we are in point 300. GMSK uses differential coding, i.e. thedirection of movement in the constellation varies depending on whetherthe bit is the same as or different than the previous one. If the bit isthe same (0→0 or 1→1), the direction of movement in the constellation iscounter-clockwise, i.e. offset 308 is performed. If the bit is different(0→1 or 1→0), the direction of movement in the constellation isclockwise, i.e. offset 310 is performed. Because in GMSK the aim is tocollect the points into two different clusters, for instance 300 and304, to minimize errors, the signal is, in addition to the previousoperation, multiplied by e^(−jkπ/4), where k is a bit index whichdepends on the location of the bit in the frame. This causes theconstellation to rotate clockwise, i.e. in the above example, the offset308 is followed by offset 312, i.e. the point stays the same.Correspondingly, offset 310 is followed by offset 314, i.e. we move topoint 304.

FIG. 4 illustrates a signal constellation of a received signal when thesignal has been transmitted by the 8-PSK modulation method. Theconstellation comprises eight points 400, 402, 404, 406, 408, 410, 412and 414. Eight points corresponding to the eight symbols transmitted in8-PSK. Each symbol comprises three bits which are Gray-coded so that thebit combinations of adjacent points always differ from each other by onebit. A possible coding is illustrated in the figure by the marked bitcombinations.

Let us examine the solution of the invention by means of the modulatorblock diagram in FIG. 5. The transmitter can, when necessary, changemodulation method. The transmitter of the invention comprises both aGMSK modulator 500 and a PSK modulator 502. The signal to be transmittedis forwarded to a first switch 504 in the modulator. The switch has twopositions. In one position, the switch 504 switches the signal to theGMSK modulator. In the other position, the switch switches the signal tothe PSK modulator. The switch receives a control signal 506 from thecontrol unit of the transmitter (not shown). The outputs of themodulators are forwarded to another switch 508 which also has twopositions. The second switch 508 is synchronized to the first switch 504so that if the first switch has switched the signal to a certainmodulator, also the second switch switches the signal path to thatmodulator. The second switch, too, receives a control signal 510 fromthe control unit of the transmitter (not shown). From the second switch508, the signal is forwarded on to the radio frequency parts.

In the transmitter of the invention, the GMSK modulator 500 can beimplemented in a manner known to those skilled in the art. FIG. 6illustrates the structure of the PSK modulator.

Let us mark the 8-PSK symbol to be transmitted with the letter S, whereS can obtain values [0, 1, . . . , 7]. The modulator comprises a coder600 in which the bits to be transmitted are first mapped as symbol S,i.e. S=Map(B2, B1, B0), where B0, B1 and B3 are three adjacent bits tobe transmitted. After this, the symbol is forwarded to a multiplier 602in which it is multiplied by the term e^(−jkπ/4), where k is a bit indexwhich depends on the location of the bit in the frame. Informationconcerning the bit index k is sent to the multiplier by the control unitof the transmitter (not shown in this figure). The multiplication causesthe rotation of the constellation. When examining FIG. 4, let us assumefor instance that three adjacent bits corresponding to the symbol 404must be transmitted after the symbol 400. First the symbol 404 is mappedand then multiplied by the term e^(−jkπ/4), which causes a 90° rotationclockwise. This takes us to point 400 which is then transmitted.Correspondingly, if the three adjacent bits to be transmittedcorresponded to the symbol 400, the rotation would result totransmitting the symbol 412.

After the multiplier 602, the signal can be presented in the formP=e^(−jkπ/4)e^(−jkπ/4), where S is the symbol to be transmitted. Fromthe multiplier 602, the signal is forwarded to a filter 604 in whichfiltering according to Gaussian distribution is preferably performed.From the filter, the signal is forwarded to a second and thirdmultiplier 606, 608 in which the signal is multiplied to radio frequencyw_(c). The signal components are combined in an adder 610 and forwardedon to the radio frequency parts.

Let us yet examine the structure of the demodulator in the receiver bymeans of the block diagram in FIG. 7. Certain advantages of the solutionof the invention are displayed in the structure of the receiver. Thedemodulator comprises a multiplier 700 in which the received signal ismultiplied by the term e^(−jkπ/4). This multiplication removes therotation of the signal constellation. From the multiplier, the signal isforwarded to an estimator 702 in which the impulse response is estimatedand in which the modulation method used can be decided. From theestimator, the signal is forwarded to a detector 704 which is preferablya Viterbi detector, for instance. In the solution of the invention, thesignal can be processed in a similar way regardless of the modulationmethod used. Without the solution of the invention, the signal mustbefore the multiplier be branched to another branch in which the signalis processed without removing the rotation. The elimination of thisbranch simplifies the implementation of the receiver.

Even though the invention has been explained in the above with referenceto examples in accordance with the attached drawings, it is obvious thatthe invention is not restricted to them but can be modified in many wayswithin the scope of the inventive idea disclosed in the attached claims.

What is claimed is:
 1. A data transmission method in which both GMSK andm-PSK modulation methods are available for a signal and wherein themodulation method used at a given time can be changed to anothermodulation method, the method comprising: when using the m-PSKmodulation method, multiplying the signal by a given coefficient thusmaking signal constellations similar regardless of the modulation methodsuch that a receiver can process the signal in a similar way regardlessof the modulation method.
 2. A method as claimed in claim 1, wherein thesignal to be transmitted comprises bursts formed by several bits andthat the coefficient given depends on bit location in the burst.
 3. Amethod as claimed in claim 1, wherein the signal to be transmittedcomprises bursts formed by several bits and that the signal to betransmitted is multiplied by a coefficient e^(−jkπ/4), where k is a bitindex.
 4. A method as claimed in claim 2, further comprising multiplyingthe signal to be transmitted by a coefficient which makes the signalconstellation rotate in the receiver like the constellation of aGMSK-modulated signal.
 5. A method as claimed in claim 1, wherein themodulation method can be changed to another modulation method withoutthe receiver knowing it.
 6. A method as claimed in claim 1, wherein themethod is applied to a cellular radio system.
 7. A radio systemcomprising: a set of receivers configured to receive a signal to betransmitted; a set of transmitters configured to transmit the signal; atleast some of which comprise both a GMSK modulator and a m-PSK modulatorto modulate the signal and a control unit adapted to select themodulation method used at a given time, wherein the m-PSK modulator ofrespective transmitters comprise a multiplier adapted to multiply thesignal to be transmitted by a given coefficient which makes the signalconstellations of the received signal similar regardless of themodulation method such that at least one receiver of said set ofreceivers can process the signal in a similar way regardless of themodulation method.
 8. A system as claimed in claim 7, wherein the signalto be transmitted comprises bursts formed of several bits and themultiplier is adapted to multiply the signal to be transmitted by acoefficient e^(−jkπ/4), where k is a bit index.
 9. A system as claimedin claim 8, wherein the signal to be transmitted comprises bursts formedof several bits and the multiplier is adapted to multiply the signal tobe transmitted by a coefficient making the signal constellation rotatein the at least one receiver like a constellation of a GMSK-modulatedsignal.
 10. A system as claimed in claim 7, wherein the radio system isa cellular radio system.